Astrocytes are the major cell type in the human brain and in recent years have emerged as critical regulators of neural circuit development, function, plasticity, and maintenance. An array of devastating neurological conditions result from astrocyte dysfunction including childhood periventricular leukomalacia, amyotrophic lateral sclerosis, and gliomas, one of the most deadly forms of cancer. Despite their fundamental importance in brain development and health, we know surprisingly little at the molecular level regarding astrocyte specification, growth, and functional interactions with neurons or synapses. We have recently made the exciting discovery that the Drosophila embryonic, larval, and adult nervous system houses a novel cell type that appears strikingly similar to mammalian astrocytes by morphological, functional, and molecular criteria. For example, fly astrocytes are only found in synapse-rich regions of the brain where they acquire a highly branched morphology, they associate closely with synapses, tile with one another to occupy unique spatial domains in the CNS, and express neurotransmitter transporters and metabolizing enzymes (e.g. EAATs, glutamine synthetase, and GABA transporters). This proposal aims to use the powerful array of molecular-genetic tools available in Drosophila, along with a number of astrocyte-specific tools we have generated, to explore fundamental questions in astrocyte biology. In this project we will (Aim1) characterize astrocyte morphology, synaptic association, polarity, and the cell-cell interactions that sculpt astrocyte architecture; (Aim 2) determine the role of the Heartless FGF receptor signaling pathway in promoting astrocyte morphogenesis and synaptic association, and (Aim 3) perform the first forward genetic screen for mutants affecting astrocyte development. We expect our work will provide exciting new insights into the molecular and cellular mechanisms regulating astrocyte development and growth control in vivo, and be highly informative in forwarding our understanding astrocyte development and dysfunction in humans.